1. Field of the Invention
The present invention relates generally to a meltable core molding system for forming hollow plastic products by molding plastic over a meltable core and, more particularly, to an improved system wherein the core molding process and overmolding process are combined in one injection molding machine to share a common clamping force and eliminate redundant machinery.
2. Description of the Related Art
Meltable core technology has long been used to make hollow plastic products. A conventional xe2x80x9clost-corexe2x80x9d system resides in a large production xe2x80x9ccellxe2x80x9d consisting of a core molding station, a core cooling station, a product molding station, and a core melting station. The cores and plastic products are typically moved from station to station with one or more robotic carriers or arms.
In general, cores of desired geometry are formed in the core molding station by injecting a molten metal alloy into a suitable core mold. The core is then ejected from the core mold and moved to the core cooling station. After cooling, the core is transferred to the product molding station where it is placed in a product mold block located between the platens of an injection-molding machine. After closing and clamping the product mold under suitable clamp tonnage, molten plastic is injected into the product mold and around the core to form an overmolded assembly. The product mold is opened after a sufficient time has passed for the plastic to cool and become suitably rigid for removal from the mold. The overmolded assembly is then ejected from the product mold and moved to the core melting station where it is placed in a hot oil tank that is at a temperature above the melting point of the core but below the melting point of the plastic. The core material melts out of the overmolded assembly to leave only the hollow plastic product. The molten core material sinks to the bottom of the hot oil tank where it is gathered and returned to the core molding station to make more cores.
Lost-core production cells are capable of producing hollow-core parts such as bicycle and wheelchair wheels, T-fittings, water meter housings, impellers, and so on. Lost-core molding is often not used for such products, however, because its considered too xe2x80x9ccomplicated,xe2x80x9d too slow, and too expensive in terms of machinery costs, energy usage, maintenance, and so on. As a result, lost-core molding is often used only as a last resort where other molding techniques are unavailable (due typically to intricate part geometries). As one example, a T-fitting is likely to be produced in a conventional injection molding machine that cycles a complicated mold with multiple core-pulls (three) that must remain in position until the part solidifies. As another example, an impeller is likely to be produced in a conventional injection molding machine that runs a mold for producing two sub-components that do not have a hollow core and must be ultrasonically welded together to complete the impeller.
The present invention makes it possible to produce complicated parts (e.g. automotive manifolds and hollow bicycle wheels), that are already being made with lost-core techniques, at less cost. The invention, moreover, makes it cost effective to produce less complicated parts such as T-fittings and impellers with lost-core techniques because, for the first time, the lost-core equipment can be cost effectively acquired, operated and maintained, and can be operated at higher production rates owing to the features of this invention and advantages associated with lost-core techniques in general (e.g. the elimination of dwell-time for parts to solidify before pulling cores in a conventional injection molding process).
The prior art lost-core cells known to these inventors have made less than optimal use of the clamp tonnage available in the injection molding machine used to form the overmold assembly. The core and product molding stations are usually separated because it is generally the companies that make core molding machines who assemble the entire cell. From their point of view, the injection molding machine, the robots, the cooling and so on, orbit around their core molding machine.
Moreover, because the molten metal is usually injected into the core mold at relatively low pressure (e.g. 500 PSI) to provide a flow-like introduction, rather than a spray-like introduction, the core molding machines are often made with correspondingly low clamp tonnage, the result being less than ideal mold closure and xe2x80x9cflashingxe2x80x9d where some of the injected metal is squeezed out of the mold cavity at the interface between two mold halves. Cores that suffer from flashing must be manually xe2x80x9cde-flashedxe2x80x9d before being loaded into the injection molding machine.
The use of separate core and product molding stations may impose a longer than otherwise reach requirement on the robot from the core molding station or even worse, require additional robots. For example, two robots may be needed, one to move a hot core to the cooling station and one to move a cool core from the cooling station to the product molding station, to remove an overmolded assembly from the product mold, to replace it with a new core from the cooling station, and to move the overmolded assembly to the core melting station.
In summary, the prior art core molding stations suffer from having the core molding station separate from the product molding station. As a result, the conventional lost-core cell is unnecessarily expensive to purchase, requires excessive floor space, consumes more energy than needed, and is relatively difficult to maintain because it requires redundant molding machines that are separately acquired, located, operated and maintained.
There remains a need, therefore, for a lost-core molding cell of simplified construction and operation.
The invention is a simplified lost-core molding system that combines otherwise redundant features such as platens and hydraulic cylinders ordinarily found in separate core molding stations and product molding stations. The preferred system locates the core mold and the product mold between the platens of an injection molding machine and applies a common clamping force to the core mold and to the product mold.
In a first aspect, the invention may be regarded as a method of producing a hollow plastic product comprising the steps of: providing molten metal; providing molten plastic; providing an injection molding machine with first and second platens; locating a core mold between the first and second platens; locating a product mold between the first and second platens; loading a core into the product mold; closing the core mold and the product mold by bringing together the first and second platens; clamping the core mold and product mold between the first and second platens with a common clamping force; injecting molten metal into the core mold to form a new core; injecting molten plastic into the product mold and around the core to form an overmolded assembly comprising a hollow plastic product that covers the core; removing the common clamping force; opening the core mold and the product mold by separating the first and second platens; removing the new core from the core mold; removing the overmolded assembly from the product mold; and loading the new core into the product mold.
In a second aspect, the invention may be regarded as molding apparatus adapted for simultaneously forming a core and an overmold assembly comprising: first and second platens that are moveable relative to one another; a core mold located between the first and second platens; a product mold located between the first and second platens; and a source of clamp tonnage for applying a common clamp tonnage to the first and second platens and, thereby, to the core mold and the product mold.